<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var month = Number(prompt('请输入月份'))
        switch(month){
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 31:
            alert("31天");
            break
            case 2:
            alert("28天");
            break
            case 4:
            case 6:
            case 9:
            case 11:
            alert("30天")
            break 
            default:
            alert('请输入1~12之间的数字')
        }
        
    </script>
</body>
</html>